While the U.S.G.A. regulates five specifications for the purposes of maintaining golf ball consistency, golf ball manufacturers constantly strive to improve alternative characteristics of the ball (i.e., spin, feel, durability, distance, sound, visibility, etc.). This is typically accomplished by altering the type of materials utilized and/or improving construction of the balls. For example, the proper choice of the materials for the cover(s) and core(s) are important in achieving certain distance, durability, and playability properties, including resilience and compression. Other important factors controlling golf ball performance include, but are not limited to, cover thickness and hardness, core stiffness (typically measured as compression), ball size, and surface configuration.
In fact, one of the principal properties driving performance of a golf ball is resilience. Resilience is generally defined as the ability of a strained body, by virtue of high yield strength and low elastic modulus, to recover its size and form following deformation. Simply stated, resilience is a measure of energy retained to the energy lost when the ball is impacted with the club. Golf balls function partly as a result of their ability to transfer kinetic energy of a moving golf club head to the golf ball and this ability is directly related to the modulus of elasticity of the various polymeric compounds that make up the components of the golf ball (in addition to the material properties of the golf club). However, because the modulus of elasticity varies with temperature, high and low temperatures will typically effect the performance of the golf ball. The coefficient of restitution (COR), which is the ratio of the outbound or rebound velocity to the incoming or inbound velocity, may be used, at least in part, as an indicator of performance at various temperatures.
For practical purposes, the optimum temperature for maximum driving distance and a “soft” feel is about 59° F. to about 95° F. Depending on the season and the climate, golf balls can be well below this optimum temperature range. Generally, the higher the temperature within a given range, the higher the modulus of elasticity, and, conversely, the lower the temperature, the lower the modulus of elasticity. In other words, as the temperature drops, golf balls generally become stiff and usually cannot be driven as far as when they are warm. In fact, golf balls stored in an unheated area may have a ball temperature of 32° F. or less, which may have a dramatic effect on the driving distance and feel of the golf ball.
One way to avoid playing with a golf ball that has a temperature outside of the optimum range is to manufacture the golf ball with a qualitative temperature indicator. Examples of golf balls having such temperature indicators are disclosed in U.S. Patent Publication No. 2003/0109329. Another way to compensate for non-optimum ball temperatures is to use a portable golf ball warmer, such as the one disclosed in U.S. Pat. No. 5,915,373. Still another attempt at reducing the effect of non-optimal temperatures on driving distance and golf ball feel is to use a golf club that compensates for various changes in stiffness of a golf ball. U.S. Pat. No. 5,899,818 discloses a golf club head having temperature-variable impact properties using a shape memory alloy that becomes stiffer at higher temperatures and more elastic at lower playing temperatures. While these methods allow a player to use a golf ball in non-optimal playing conditions and may allow the golfer to achieve adequate distance, they require special equipment to do so. And, because COR is generally a function of the composition of the components of the golf ball, it would be advantageous to find a particular material or blend of materials that provides desirable COR and, thus, desirable performance at non-optimal temperatures.
In addition, as indicated above, compression is another important property involved in the overall performance of a golf ball. The compression of a ball will influence the sound or “click” produced when the ball is properly hit. Similarly, compression can effect the “feel” of the ball (i.e., hard or soft responsive feel), particularly in chipping and putting. Moreover, while compression of itself has little bearing on the distance performance of a ball, compression can affect the playability of the ball on striking.
The degree of compression of a ball against the club face and the softness of the cover strongly influences the resultant spin rate. For example, a softer cover will typically produce a higher spin rate than a harder cover whereas a harder core will produce a higher spin rate than a softer core because, at impact, a hard core serves to compress the cover of the ball against the face of the club to a much greater degree than a soft core. In contrast, when a softer core is used, the cover is under much less compressive stress than when a harder core is used resulting in less intimate contact with the clubface and lower spin rates. Thus, it would be advantageous to find the right combination of materials for use in a golf ball to deliver optimal compression.
Furthermore, cover hardness and thickness are important in producing the distance, playability and durability properties of a golf ball. As mentioned above, cover hardness directly affects the resilience and thus distance characteristics of a ball. All things being equal, harder covers produce higher resilience. This is because soft materials detract from resilience by absorbing some of the impact energy as the material is compressed on striking. Therefore, it would be advantageous to also achieve an optimal combination of layer hardness and thickness for use in golf balls.
For example, ionomeric resins have been used to achieve durability in golf balls. However, some of the advantages gained in increased durability have been offset to some degree by decreases in playability due to the hardness of the material. As a result, a great deal of research continues in order to develop golf ball components exhibiting not only the improved impact resistance and carrying distance properties produced by the “hard” ionomeric resins, but also the playability (i.e., “spin”, “feel”, etc.) characteristics previously associated with the “soft” balata covers, properties still desired by the more skilled golfer. In addition, various golf ball compositions with purported improved light stability and durability have been disclosed. For example, U.S. Pat. No. 6,458,307 is directed to thermoplastic cover materials with a reported improvement in light stability, cut resistance, and abrasion resistance. U.S. Pat. No. 6,369,125 relates to crosslinkable thermoplastic compositions that can be melted and reformed and also have improved scuff and cut resistance over conventional balata covers. However, like ionomeric resins, these materials have fallen short in several respects in achieving optimal performance characteristics.
In addition to the properties discussed above, there remains a need in the art to overcome the deficiencies of prior art materials with respect to degradation at elevated temperatures. For example, wrinkling of the golf ball component may occur at about 50° C. In fact, even the most advanced light stable polyurethane and polyurea compositions have been shown to be susceptible to heat degradation during additional processing steps, e.g., coating and marking, and upon storage in non-optimal temperatures. Several manufacturers have attempted to compensate for any golf ball cover degradation upon application of heat by using coatings having contraction and expansion properties. For example, U.S. Pat. No. 5,816,943 is directed to a coating having a higher heat resistance than the cover material to prevent shallowing of dimples or dulling of dimple edges upon the coating application. However, these efforts obviously require an additional step or processing time.
In sum, while past efforts by manufacturers compensate for many problems associated with golf play during non-optimal conditions, no method of material has addressed all of the problems at once. In addition, most of the methods used to compensate for extreme temperature conditions involve the use of a special indicator, club, or warmer. Therefore, there remains a continuing need for novel compositions that solve temperature-related problems of conventionally-formed golf balls and golf clubs, e.g., resiliency and material degradation at non-optimal temperatures. In particular, it would be advantageous to provide a composition using materials that provides heat resistance and impact strength, as well as improved resiliency, to golf ball and club components formed therefrom.